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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="RUNTIME-CONFIG-WAL"
>18.5. Write Ahead Log</A
></H1
><P
>    See also <A
HREF="wal-configuration.html"
>Section 29.4</A
> for details on WAL
    and checkpoint tuning.
   </P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-WAL-SETTINGS"
>18.5.1. Settings</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-WAL-LEVEL"
></A
><TT
CLASS="VARNAME"
>wal_level</TT
> (<TT
CLASS="TYPE"
>enum</TT
>)</DT
><DD
><P
>        <TT
CLASS="VARNAME"
>wal_level</TT
> determines how much information is written
        to the WAL. The default value is <TT
CLASS="LITERAL"
>minimal</TT
>, which writes
        only the information needed to recover from a crash or immediate
        shutdown. <TT
CLASS="LITERAL"
>archive</TT
> adds logging required for WAL archiving,
        and <TT
CLASS="LITERAL"
>hot_standby</TT
> further adds information required to run
        read-only queries on a standby server.
        This parameter can only be set at server start.
       </P
><P
>        In <TT
CLASS="LITERAL"
>minimal</TT
> level, WAL-logging of some bulk
        operations can be safely skipped, which can make those
        operations much faster (see <A
HREF="populate.html#POPULATE-PITR"
>Section 14.4.7</A
>).
        Operations in which this optimization can be applied include:
        <P
></P
><TABLE
BORDER="0"
><TBODY
><TR
><TD
><TT
CLASS="COMMAND"
>CREATE TABLE AS</TT
></TD
></TR
><TR
><TD
><TT
CLASS="COMMAND"
>CREATE INDEX</TT
></TD
></TR
><TR
><TD
><TT
CLASS="COMMAND"
>CLUSTER</TT
></TD
></TR
><TR
><TD
><TT
CLASS="COMMAND"
>COPY</TT
> into tables that were created or truncated in the same
         transaction</TD
></TR
></TBODY
></TABLE
><P
></P
>
        But minimal WAL does not contain
        enough information to reconstruct the data from a base backup and the
        WAL logs, so either <TT
CLASS="LITERAL"
>archive</TT
> or <TT
CLASS="LITERAL"
>hot_standby</TT
>
        level must be used to enable
        WAL archiving (<A
HREF="runtime-config-wal.html#GUC-ARCHIVE-MODE"
>archive_mode</A
>) and streaming
        replication.
       </P
><P
>        In <TT
CLASS="LITERAL"
>hot_standby</TT
> level, the same information is logged as
        with <TT
CLASS="LITERAL"
>archive</TT
>, plus information needed to reconstruct
        the status of running transactions from the WAL. To enable read-only
        queries on a standby server, <TT
CLASS="VARNAME"
>wal_level</TT
> must be set to
        <TT
CLASS="LITERAL"
>hot_standby</TT
> on the primary, and
        <A
HREF="runtime-config-replication.html#GUC-HOT-STANDBY"
>hot_standby</A
> must be enabled in the standby. It is
        thought that there is
        little measurable difference in performance between using
        <TT
CLASS="LITERAL"
>hot_standby</TT
> and <TT
CLASS="LITERAL"
>archive</TT
> levels, so feedback
        is welcome if any production impacts are noticeable.
       </P
></DD
><DT
><A
NAME="GUC-FSYNC"
></A
><TT
CLASS="VARNAME"
>fsync</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
>        If this parameter is on, the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> server
        will try to make sure that updates are physically written to
        disk, by issuing <CODE
CLASS="FUNCTION"
>fsync()</CODE
> system calls or various
        equivalent methods (see <A
HREF="runtime-config-wal.html#GUC-WAL-SYNC-METHOD"
>wal_sync_method</A
>).
        This ensures that the database cluster can recover to a
        consistent state after an operating system or hardware crash.
       </P
><P
>        While turning off <TT
CLASS="VARNAME"
>fsync</TT
> is often a performance
        benefit, this can result in unrecoverable data corruption in
        the event of a power failure or system crash.  Thus it
        is only advisable to turn off <TT
CLASS="VARNAME"
>fsync</TT
> if
        you can easily recreate your entire database from external
        data.
       </P
><P
>        Examples of safe circumstances for turning off
        <TT
CLASS="VARNAME"
>fsync</TT
> include the initial loading of a new
        database cluster from a backup file, using a database cluster
        for processing a batch of data after which the database
        will be thrown away and recreated,
        or for a read-only database clone which
        gets recreated frequently and is not used for failover.  High
        quality hardware alone is not a sufficient justification for
        turning off <TT
CLASS="VARNAME"
>fsync</TT
>.
       </P
><P
>        In many situations, turning off <A
HREF="runtime-config-wal.html#GUC-SYNCHRONOUS-COMMIT"
>synchronous_commit</A
>
        for noncritical transactions can provide much of the potential
        performance benefit of turning off <TT
CLASS="VARNAME"
>fsync</TT
>, without
        the attendant risks of data corruption.
       </P
><P
>        <TT
CLASS="VARNAME"
>fsync</TT
> can only be set in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
        file or on the server command line.
        If you turn this parameter off, also consider turning off
        <A
HREF="runtime-config-wal.html#GUC-FULL-PAGE-WRITES"
>full_page_writes</A
>.
       </P
></DD
><DT
><A
NAME="GUC-SYNCHRONOUS-COMMIT"
></A
><TT
CLASS="VARNAME"
>synchronous_commit</TT
> (<TT
CLASS="TYPE"
>enum</TT
>)</DT
><DD
><P
>        Specifies whether transaction commit will wait for WAL records
        to be written to disk before the command returns a <SPAN
CLASS="QUOTE"
>"success"</SPAN
>
        indication to the client.  Valid values are <TT
CLASS="LITERAL"
>on</TT
>,
        <TT
CLASS="LITERAL"
>local</TT
>, and <TT
CLASS="LITERAL"
>off</TT
>.  The default, and safe, value
        is <TT
CLASS="LITERAL"
>on</TT
>.  When <TT
CLASS="LITERAL"
>off</TT
>, there can be a delay between
        when success is reported to the client and when the transaction is
        really guaranteed to be safe against a server crash.  (The maximum
        delay is three times <A
HREF="runtime-config-wal.html#GUC-WAL-WRITER-DELAY"
>wal_writer_delay</A
>.)  Unlike
        <A
HREF="runtime-config-wal.html#GUC-FSYNC"
>fsync</A
>, setting this parameter to <TT
CLASS="LITERAL"
>off</TT
>
        does not create any risk of database inconsistency: an operating
        system or database crash might
        result in some recent allegedly-committed transactions being lost, but
        the database state will be just the same as if those transactions had
        been aborted cleanly.  So, turning <TT
CLASS="VARNAME"
>synchronous_commit</TT
> off
        can be a useful alternative when performance is more important than
        exact certainty about the durability of a transaction.  For more
        discussion see <A
HREF="wal-async-commit.html"
>Section 29.3</A
>.
       </P
><P
>        If <A
HREF="runtime-config-replication.html#GUC-SYNCHRONOUS-STANDBY-NAMES"
>synchronous_standby_names</A
> is set, this
        parameter also controls whether or not transaction commit will wait
        for the transaction's WAL records to be flushed to disk and replicated
        to the standby server.  The commit wait will last until a reply from
        the current synchronous standby indicates it has written the commit
        record of the transaction to durable storage.  If synchronous
        replication is in use, it will normally be sensible either to wait
        both for WAL records to reach both the local and remote disks, or
        to allow the transaction to commit asynchronously.  However, the
        special value <TT
CLASS="LITERAL"
>local</TT
> is available for transactions that
        wish to wait for local flush to disk, but not synchronous replication.
       </P
><P
>        This parameter can be changed at any time; the behavior for any
        one transaction is determined by the setting in effect when it
        commits.  It is therefore possible, and useful, to have some
        transactions commit synchronously and others asynchronously.
        For example, to make a single multistatement transaction commit
        asynchronously when the default is the opposite, issue <TT
CLASS="COMMAND"
>SET
        LOCAL synchronous_commit TO OFF</TT
> within the transaction.
       </P
></DD
><DT
><A
NAME="GUC-WAL-SYNC-METHOD"
></A
><TT
CLASS="VARNAME"
>wal_sync_method</TT
> (<TT
CLASS="TYPE"
>enum</TT
>)</DT
><DD
><P
>        Method used for forcing WAL updates out to disk.
        If <TT
CLASS="VARNAME"
>fsync</TT
> is off then this setting is irrelevant,
        since WAL file updates will not be forced out at all.
        Possible values are:
       </P
><P
></P
><UL
><LI
><P
>         <TT
CLASS="LITERAL"
>open_datasync</TT
> (write WAL files with <CODE
CLASS="FUNCTION"
>open()</CODE
> option <TT
CLASS="SYMBOL"
>O_DSYNC</TT
>)
        </P
></LI
><LI
><P
>         <TT
CLASS="LITERAL"
>fdatasync</TT
> (call <CODE
CLASS="FUNCTION"
>fdatasync()</CODE
> at each commit)
        </P
></LI
><LI
><P
>         <TT
CLASS="LITERAL"
>fsync</TT
> (call <CODE
CLASS="FUNCTION"
>fsync()</CODE
> at each commit)
        </P
></LI
><LI
><P
>         <TT
CLASS="LITERAL"
>fsync_writethrough</TT
> (call <CODE
CLASS="FUNCTION"
>fsync()</CODE
> at each commit, forcing write-through of any disk write cache)
        </P
></LI
><LI
><P
>         <TT
CLASS="LITERAL"
>open_sync</TT
> (write WAL files with <CODE
CLASS="FUNCTION"
>open()</CODE
> option <TT
CLASS="SYMBOL"
>O_SYNC</TT
>)
        </P
></LI
></UL
><P
>        The <TT
CLASS="LITERAL"
>open_</TT
>* options also use <TT
CLASS="LITERAL"
>O_DIRECT</TT
> if available.
        Not all of these choices are available on all platforms.
        The default is the first method in the above list that is supported
        by the platform, except that <TT
CLASS="LITERAL"
>fdatasync</TT
> is the default on
        Linux.  The default is not necessarily ideal; it might be
        necessary to change this setting or other aspects of your system
        configuration in order to create a crash-safe configuration or
        achieve optimal performance.
        These aspects are discussed in <A
HREF="wal-reliability.html"
>Section 29.1</A
>.
        This parameter can only be set in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
        file or on the server command line.
       </P
></DD
><DT
><A
NAME="GUC-FULL-PAGE-WRITES"
></A
><TT
CLASS="VARNAME"
>full_page_writes</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
>        When this parameter is on, the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> server
        writes the entire content of each disk page to WAL during the
        first modification of that page after a checkpoint.
        This is needed because
        a page write that is in process during an operating system crash might
        be only partially completed, leading to an on-disk page
        that contains a mix of old and new data.  The row-level change data
        normally stored in WAL will not be enough to completely restore
        such a page during post-crash recovery.  Storing the full page image
        guarantees that the page can be correctly restored, but at the price
        of increasing the amount of data that must be written to WAL.
        (Because WAL replay always starts from a checkpoint, it is sufficient
        to do this during the first change of each page after a checkpoint.
        Therefore, one way to reduce the cost of full-page writes is to
        increase the checkpoint interval parameters.)
       </P
><P
>        Turning this parameter off speeds normal operation, but
        might lead to either unrecoverable data corruption, or silent
        data corruption, after a system failure. The risks are similar to turning off
        <TT
CLASS="VARNAME"
>fsync</TT
>, though smaller, and it should be turned off
        only based on the same circumstances recommended for that parameter.
       </P
><P
>        Turning off this parameter does not affect use of
        WAL archiving for point-in-time recovery (PITR)
        (see <A
HREF="continuous-archiving.html"
>Section 24.3</A
>).
       </P
><P
>        This parameter can only be set in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
        file or on the server command line.
        The default is <TT
CLASS="LITERAL"
>on</TT
>.
       </P
></DD
><DT
><A
NAME="GUC-WAL-BUFFERS"
></A
><TT
CLASS="VARNAME"
>wal_buffers</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>        The amount of shared memory used for WAL data that has not yet been
        written to disk.  The default setting of -1 selects a size equal to
        1/32nd (about 3%) of <A
HREF="runtime-config-resource.html#GUC-SHARED-BUFFERS"
>shared_buffers</A
>, but not less
        than <TT
CLASS="LITERAL"
>64kB</TT
> nor more than the size of one WAL
        segment, typically <TT
CLASS="LITERAL"
>16MB</TT
>.  This value can be set
        manually if the automatic choice is too large or too small,
        but any positive value less than <TT
CLASS="LITERAL"
>32kB</TT
> will be
        treated as <TT
CLASS="LITERAL"
>32kB</TT
>.
        This parameter can only be set at server start.
       </P
><P
>        The contents of the WAL buffers are written out to disk at every
        transaction commit, so extremely large values are unlikely to
        provide a significant benefit.  However, setting this value to at
        least a few megabytes can improve write performance on a busy
        server where many clients are committing at once.  The auto-tuning
        selected by the default setting of -1 should give reasonable
        results in most cases.
       </P
><P
>        Increasing this parameter might cause <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
        to request more <SPAN
CLASS="SYSTEMITEM"
>System V</SPAN
> shared
        memory than your operating system's default configuration
        allows. See <A
HREF="kernel-resources.html#SYSVIPC"
>Section 17.4.1</A
> for information on how to
        adjust those parameters, if necessary.
       </P
></DD
><DT
><A
NAME="GUC-WAL-WRITER-DELAY"
></A
><TT
CLASS="VARNAME"
>wal_writer_delay</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>        Specifies the delay between activity rounds for the WAL writer.
        In each round the writer will flush WAL to disk. It then sleeps for
        <TT
CLASS="VARNAME"
>wal_writer_delay</TT
> milliseconds, and repeats.  The default
        value is 200 milliseconds (<TT
CLASS="LITERAL"
>200ms</TT
>).  Note that on many
        systems, the effective resolution of sleep delays is 10 milliseconds;
        setting <TT
CLASS="VARNAME"
>wal_writer_delay</TT
> to a value that is not a multiple
        of 10 might have the same results as setting it to the next higher
        multiple of 10. This parameter can only be set in the
        <TT
CLASS="FILENAME"
>postgresql.conf</TT
> file or on the server command line.
       </P
></DD
><DT
><A
NAME="GUC-COMMIT-DELAY"
></A
><TT
CLASS="VARNAME"
>commit_delay</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>        When the commit data for a transaction is flushed to disk, any
        additional commits ready at that time are also flushed out.
        <TT
CLASS="VARNAME"
>commit_delay</TT
> adds a time delay, set in
        microseconds, before a transaction attempts to
        flush the WAL buffer out to disk.  A nonzero delay can allow more
        transactions to be committed with only one flush operation, if
        system load is high enough that additional transactions become
        ready to commit within the given interval. But the delay is
        just wasted if no other transactions become ready to
        commit. Therefore, the delay is only performed if at least
        <TT
CLASS="VARNAME"
>commit_siblings</TT
> other transactions are
        active at the instant that a server process has written its
        commit record.
        The default <TT
CLASS="VARNAME"
>commit_delay</TT
> is zero (no delay).
        Since all pending commit data will be written at every flush
        regardless of this setting, it is rare that adding delay
        by increasing this parameter will actually improve performance.
       </P
></DD
><DT
><A
NAME="GUC-COMMIT-SIBLINGS"
></A
><TT
CLASS="VARNAME"
>commit_siblings</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>        Minimum number of concurrent open transactions to require
        before performing the <TT
CLASS="VARNAME"
>commit_delay</TT
> delay. A larger
        value makes it more probable that at least one other
        transaction will become ready to commit during the delay
        interval. The default is five transactions.
       </P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-WAL-CHECKPOINTS"
>18.5.2. Checkpoints</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-CHECKPOINT-SEGMENTS"
></A
><TT
CLASS="VARNAME"
>checkpoint_segments</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>        Maximum number of log file segments between automatic WAL
        checkpoints (each segment is normally 16 megabytes). The default
        is three segments.  Increasing this parameter can increase the
        amount of time needed for crash recovery.
        This parameter can only be set in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
        file or on the server command line.
       </P
></DD
><DT
><A
NAME="GUC-CHECKPOINT-TIMEOUT"
></A
><TT
CLASS="VARNAME"
>checkpoint_timeout</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>        Maximum time between automatic WAL checkpoints, in
        seconds. The default is five minutes (<TT
CLASS="LITERAL"
>5min</TT
>).
        Increasing this parameter can increase the amount of time needed
        for crash recovery.
        This parameter can only be set in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
        file or on the server command line.
       </P
></DD
><DT
><A
NAME="GUC-CHECKPOINT-COMPLETION-TARGET"
></A
><TT
CLASS="VARNAME"
>checkpoint_completion_target</TT
> (<TT
CLASS="TYPE"
>floating point</TT
>)</DT
><DD
><P
>        Specifies the target of checkpoint completion, as a fraction of
        total time between checkpoints. The default is 0.5.

        This parameter can only be set in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
        file or on the server command line.
       </P
></DD
><DT
><A
NAME="GUC-CHECKPOINT-WARNING"
></A
><TT
CLASS="VARNAME"
>checkpoint_warning</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>        Write a message to the server log if checkpoints caused by
        the filling of checkpoint segment files happen closer together
        than this many seconds (which suggests that
        <TT
CLASS="VARNAME"
>checkpoint_segments</TT
> ought to be raised).  The default is
        30 seconds (<TT
CLASS="LITERAL"
>30s</TT
>).  Zero disables the warning.
        This parameter can only be set in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
        file or on the server command line.
       </P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-WAL-ARCHIVING"
>18.5.3. Archiving</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-ARCHIVE-MODE"
></A
><TT
CLASS="VARNAME"
>archive_mode</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
>        When <TT
CLASS="VARNAME"
>archive_mode</TT
> is enabled, completed WAL segments
        are sent to archive storage by setting
        <A
HREF="runtime-config-wal.html#GUC-ARCHIVE-COMMAND"
>archive_command</A
>.
        <TT
CLASS="VARNAME"
>archive_mode</TT
> and <TT
CLASS="VARNAME"
>archive_command</TT
> are
        separate variables so that <TT
CLASS="VARNAME"
>archive_command</TT
> can be
        changed without leaving archiving mode.
        This parameter can only be set at server start.
        <TT
CLASS="VARNAME"
>archive_mode</TT
> cannot be enabled when
        <TT
CLASS="VARNAME"
>wal_level</TT
> is set to <TT
CLASS="LITERAL"
>minimal</TT
>.
       </P
></DD
><DT
><A
NAME="GUC-ARCHIVE-COMMAND"
></A
><TT
CLASS="VARNAME"
>archive_command</TT
> (<TT
CLASS="TYPE"
>string</TT
>)</DT
><DD
><P
>        The shell command to execute to archive a completed WAL file
        segment.  Any <TT
CLASS="LITERAL"
>%p</TT
> in the string is
        replaced by the path name of the file to archive, and any
        <TT
CLASS="LITERAL"
>%f</TT
> is replaced by only the file name.
        (The path name is relative to the working directory of the server,
        i.e., the cluster's data directory.)
        Use <TT
CLASS="LITERAL"
>%%</TT
> to embed an actual <TT
CLASS="LITERAL"
>%</TT
> character in the
        command.  It is important for the command to return a zero
        exit status only if it succeeds. For more information see
        <A
HREF="continuous-archiving.html#BACKUP-ARCHIVING-WAL"
>Section 24.3.1</A
>.
       </P
><P
>        This parameter can only be set in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
        file or on the server command line.  It is ignored unless
        <TT
CLASS="VARNAME"
>archive_mode</TT
> was enabled at server start.
        If <TT
CLASS="VARNAME"
>archive_command</TT
> is an empty string (the default) while
        <TT
CLASS="VARNAME"
>archive_mode</TT
> is enabled, WAL archiving is temporarily
        disabled, but the server continues to accumulate WAL segment files in
        the expectation that a command will soon be provided.  Setting
        <TT
CLASS="VARNAME"
>archive_command</TT
> to a command that does nothing but
        return true, e.g. <TT
CLASS="LITERAL"
>/bin/true</TT
> (<TT
CLASS="LITERAL"
>REM</TT
> on
        Windows), effectively disables
        archiving, but also breaks the chain of WAL files needed for
        archive recovery, so it should only be used in unusual circumstances.
       </P
></DD
><DT
><A
NAME="GUC-ARCHIVE-TIMEOUT"
></A
><TT
CLASS="VARNAME"
>archive_timeout</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>        The <A
HREF="runtime-config-wal.html#GUC-ARCHIVE-COMMAND"
>archive_command</A
> is only invoked for
        completed WAL segments. Hence, if your server generates little WAL
        traffic (or has slack periods where it does so), there could be a
        long delay between the completion of a transaction and its safe
        recording in archive storage.  To limit how old unarchived
        data can be, you can set <TT
CLASS="VARNAME"
>archive_timeout</TT
> to force the
        server to switch to a new WAL segment file periodically.  When this
        parameter is greater than zero, the server will switch to a new
        segment file whenever this many seconds have elapsed since the last
        segment file switch, and there has been any database activity,
        including a single checkpoint.  (Increasing
        <TT
CLASS="VARNAME"
>checkpoint_timeout</TT
> will reduce unnecessary
        checkpoints on an idle system.)
        Note that archived files that are closed early
        due to a forced switch are still the same length as completely full
        files.  Therefore, it is unwise to use a very short
        <TT
CLASS="VARNAME"
>archive_timeout</TT
> &mdash; it will bloat your archive
        storage.  <TT
CLASS="VARNAME"
>archive_timeout</TT
> settings of a minute or so are
        usually reasonable.  You should consider using streaming replication,
        instead of archiving, if you want data to be copied off the master
        server more quickly than that.
        This parameter can only be set in the
        <TT
CLASS="FILENAME"
>postgresql.conf</TT
> file or on the server command line.
       </P
></DD
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